add default initializer to let variables within loop bodies and fix excessive break replacement inside of switches in let scoping - fixes #255

lib/6to5/transformation/transformers/es6-let-scoping.js

@@ -91,6 +91,9 @@ LetScoping.prototype.run = function () {
   // remap all let references that exist in upper scopes to their uid
   this.remap();
 
+  // add default initializer to let variables in loop bodys
+  this.initialiseLoopLets();
+
   // this is a block within a `Function` so we can safely leave it be
   if (t.isFunction(this.parent)) return this.noClosure();
 
@@ -106,7 +109,7 @@ LetScoping.prototype.run = function () {
 
   // if we're inside of a for loop then we search to see if there are any
   // `break`s, `continue`s, `return`s etc
-  this.has = this.checkFor();
+  this.has = this.checkLoop();
 
   // hoist var references to retain scope
   this.hoistVarDeclarations();
@@ -244,6 +247,30 @@ LetScoping.prototype.getInfo = function () {
   return opts;
 };
 
+/**
+ * Any let variable declared within a loop body need to have an initializer or
+ * else they'll be hoisted and subsequent iterations of the loop will have a
+ * previous state. This function adds a default initializer of `undefined` to
+ * those variables.
+ */
+
+LetScoping.prototype.initialiseLoopLets = function () {
+  var loopParent = this.loopParent;
+  if (!loopParent) return;
+
+  traverse(this.block, function (node) {
+    if (t.isFunction(node) || t.isLoop(node)) {
+      return false;
+    }
+
+    if (isLet(node)) {
+      _.each(node.declarations, function (declar) {
+        declar.init = declar.init || t.identifier("undefined");
+      });
+    }
+  });
+};
+
 /**
  * If we're inside of a `For*Statement` then traverse it and check if it has one
  * of the following node types `ReturnStatement`, `BreakStatement`,
@@ -253,22 +280,26 @@ LetScoping.prototype.getInfo = function () {
  * @returns {Object}
  */
 
-LetScoping.prototype.checkFor = function () {
+LetScoping.prototype.checkLoop = function () {
   var has = {
     hasContinue: false,
     hasReturn:   false,
     hasBreak:    false,
   };
 
-  traverse(this.block, function (node) {
+  var loopParent = this.loopParent;
+
+  traverse(this.block, function (node, parent) {
     var replace;
 
-    if (t.isFunction(node) || t.isLoop(node) || t.isSwitchStatement(node)) {
+    if (t.isFunction(node) || t.isLoop(node)) {
       return false;
     }
 
     if (node && !node.label) {
       if (t.isBreakStatement(node)) {
+        if (t.isSwitchCase(parent)) return;
+
         has.hasBreak = true;
         replace = t.returnStatement(t.literal("break"));
       } else if (t.isContinueStatement(node)) {